Current digital cellular telephone systems such as GSM (Global System for Mobile communications) were designed with an emphasis on voice communications. Data is normally transmitted between a mobile station (MS) and a base station subsystem (BSS) over the air interface using the so called circuit switched transmission mode where a physical channel, i.e. a series of regularly spaced time slots on one or more frequencies, is reserved for the duration of the call. For voice communications, where the stream of information to be transmitted is relatively continuous, the circuit switched transmission mode is reasonably efficient. However, during data calls, e.g, internet access, the data stream is ‘bursty’ and the long term reservation of a physical channel in the circuit switched mode represents an uneconomic use of the air interface.
Given that the demand for data services with digital cellular telephone systems is increasing rapidly, a new GSM based service known as the General Packet Radio Service (GPRS) is currently being standardised by the European Telecommunications Standards Institute (ETSI) and is defined in overall terms in GSM 03.60. GPRS provides for the dynamic allocation of physical channels for data transmission. That is to say that a physical channel is allocated to a particular MS to BSS link only when there is data to be transmitted. The unnecessary reservation of physical channels when there is no data to be transmitted is avoided.
GPRS is intended to operate in conjunction with conventional GSM circuit switched transmission to efficiently use the air interface for both data and voice communications. GPRS will therefore use the basic channel structure defined for GSM. In GSM, a given frequency band is divided in the time domain into a succession of frames, known as TDMA (Time Division Multiplexed Access) frames. The length of TDMA frame is 4.615 ms. Each TDMA frame is in turn divided into eight consecutive slots of equal duration. In the conventional circuit switched transmission mode, when a call is initiated, a physical channel is defined for that call by reserving a given time slot (1 to 8) in each of a succession of TDMA frames. A series of four consecutive time slots on a physical channel is known as a radio block and represents the shortest transmission unit for packet switched data on a physical channel. Physical channels are similarly defined for conveying signalling information. With the introduction of GPRS, physical channels will be dynamically assigned for either switched circuit transmission mode or for packet switched transmission mode. When the network requirement for switched circuit transmission mode is high, a large number of physical channels may be reserved for that mode. On the other hand, when demand for GPRS transmission is high, a large number of physical channels may be reserved for that mode. In addition, a high speed packet switched transmission channel may be provided by assigning two or more slots in each of a succession of TDMA frames to a single MS.
The GPRS radio interface for GSM Phase 2+ (GSM 04.65) can be modelled as a hierarchy of logical layers with specific functions as shown in FIG. 1, where the mobile station (MS) and the network have identical layers which communicate via the MS/network interface Um. Each layer formats data received from the neighbouring layer, with received data passing from the bottom to the top layer and data for transmission passing from the top to the bottom layer.
At the top layer are a number of packet data protocols (PDPs). Certain of these PDPs are point-to-point protocols (PTPs) adapted for sending packet data from one MS to another MS, or from one MS to a fixed terminal. Examples of PTP protocols are IP (internet access protocol) and X.25. The PDPs all use a common subnetwork dependent convergence protocol (SNDCP) which, as its name suggests, translates (or ‘converges’) the different PDPs into a common form (composed of SNDCP units) suitable for further processing in a transparent way. This architecture means that new PDPs may be developed in the future which can be readily incorporated into the existing GPRS architecture.
The SNDCP defines multiplexing and segmentation of user data, data compression, TCP/IP header compression, as well as transmission according to the requested quality of service. SNDCP units are about 1600 octets and comprise an address field which contains a network service access point identifier (NSAPI) which is used to identify the endpoint connection, e.g. IP, X.25. Each MS may be assigned a set of NSAPIs independently of the other MSs.
Also on the top layer are other GPRS end point protocols such as SMS and signalling (L3M). Each SNDCP (or other GPRS end point protocol) unit is carried by one logical link control (LLC) frame over the radio interface. The LLC frames are formulated in the LLC layer (GSM 04.64) and include a header frame with numbering and temporary addressing fields, a variable length information field, and a frame check sequence. More particularly, the addressing fields include a service access point identifier (SAPI) which is used to identify a specific connection endpoint (and its relative priority and Quality of Service (QoS)) on the network side and the user side of the LLC interface. One connection endpoint is the SNDCP. Other endpoints include the short message service (SMS) and management layer (L3M). The LLC layer provides a convergence protocol for these different endpoint protocols. SAPIs are allocated permanently and are common to all MSs.
The Radio Link Control (RLC) layer defines amongst other things the procedures for segmenting and re-assembling Logical Link Control layer PDUs (LLC-PDU) into RLC Data Blocks, and for retransmission of unsuccessfully delivered RLC blocks. The Medium Access Control (MAC) layer operates above the Phys. Link layer (see below) and defines the procedures that enable multiple MSs to share a common transmission medium. The MAC function arbitrates between multiple MSs attempting to transmit simultaneously and provides collision avoidance, detection and recovery procedures.
The physical link layer (Phys. Link) provides a physical channel between the MS and the network). The physical RF layer (Phys. RF) specifies amongst other things the carrier frequencies and GSM radio channel structures, modulation of the GSM channels, and transmitter/receiver characteristics.
For GPRS transmission, three different mobility management states are defined: IDLE, STANDBY, and READY. An IDLE state MS is not GPRS ‘attached’ and so the network is not aware of this MS. However, the MS is listening to broadcast control messages, for example, to determine network cell selection. A STANDBY state MS is GPRS attached and it's location (routing area) is tracked by the network. However, there is no data being transmitted. A MS is in a READY state when it is transmitting data and for a short while after. A READY state MS is therefore also tracked by the network. As currently proposed, there are 16 unique NSAPI codes available for identifying PDPs. The NSAPI codes are assigned dynamically by the network so that a MS must be in either the STANDBY state or the READY state to be aware of the allocated codes. As currently proposed, an IDLE state MS cannot receive transmissions in any PDP. For PDPs such as IP and X.25 this does not present a problem as the MS will always be in either the STANDBY or READY state when such transmissions are taking place.
In addition to PTPs, it is likely that future releases of GSM will specify other PDPs and in particular point-to-multipoint (PTM) transfer where data is transmitted to a group of MSs (PTM-G, point-to-multipoint-groupcall) or to all mobiles in an area (PTM-M, point-to-multipoint-multicast). The uses of such PDPs include operator announcements, advertisements, and specific information transfer such as football results, news etc. PTP-G is similar to PTP in so far as a MS must be in either the STANDBY or READY state to receive a transmission. However, a hitherto unrecognised problem arises with PTM-M due to the need (defined in GSM 03.60) for a MS to receive PTM-M transmissions in all states including the IDLE state. As no PDP contexts are active when a MS is in the IDLE state, and the allocation of NSAPI codes by the network is dynamic, an IDLE MS cannot allocate the correct NSAPI code to a PTM-M and therefore cannot receive a PTM-M.
Whilst the above discussion of GPRS has been concerned with GSM, it is noted that GPRS has a much wider applicability. For example, by changing only the low level radio protocol, GPRS may be adapted to the proposed third generation standard UMTS (Universal Mobile Telecommunication System)